The present invention relates to a catalyst for nitrogen oxide removal and a method for removing nitrogen oxides.
Nitrogen oxides (nitrogen monoxide, nitrogen dioxide, nitrous oxide, etc.), incomplete combustion components (carbon monoxide, hydrogen, hydrocarbons, etc.), and water are major components of exhaust gases exhausted from various internal-combustion engines, combustion apparatuses, etc. Among them, nitrogen oxides are not only harmful to the human body but also one of principal substances causative of acid rain. Thus, countermeasures against nitrogen oxides are desired.
Researches on catalysts for reducing and removing nitrogen oxides using a reducing agent as such a countermeasure are now under study everywhere. The selective catalyst reduction method using ammonia and the three-way catalyst method in particular are known among methods of removing nitrogen oxides from exhaust gases.
The ammoniac selective reduction method is a method wherein nitrogen oxides are reduced and removed using ammonia as the reducing agent and a catalyst comprising vanadium oxide and titanium oxide as the basic material. This method is however restricted in uses and places wherein it can be employed because ammonia is intractable.
On the other hand, the three-way catalyst method is a method wherein nitrogen oxides are reduced and removed using hydrocarbons, carbon monoxide, etc. present in exhaust gases as the reducing agent. According to this method, however, a reaction of the reducing agents with oxygen proceeds preferentially to the reaction of the reducing agents with nitrogen oxides when a high concentration (about 1 to 10%) of oxygen exists in an exhaust gas, leading to the problem that nitrogen oxides cannot effectively be reduced and removed.
Accordingly, in order to solve these technical problems, there is proposed a method of selectively reducing nitrogen oxides using a hydrocarbon as the reducing agent and a catalyst comprising a copper ion-exchanged zeolite, aluminum oxide, etc. as the basic active substance. According to this method, nitrogen oxides can be reduced and removed even if oxygen is coexistent.
However, the foregoing selective reduction method involves such problems that the reaction temperature region where reduction is possible is usually as high as 400.degree. C. or above, and that the removal performance is notably lowered if moisture is contained in an exhaust gas. In this respect, there is room for improvements.